1. Field of the Invention
The present invention relates to magnetic bearing systems, and more specifically, it relates to axial and transverse stabilizers for such systems.
2. Description of Related Art
Passive magnetic bearing systems developed at Lawrence Livermore National Laboratory (LLNL) have been described in several U.S. patents, e.g., U.S. Pat. No. 5,495,223 “Dynamically Stable Magnetic Suspension System,” and U.S. Pat. No. 5,847,480, “Passive Magnetic Bearing Element with Minimal Power Losses.” Studies now underway at the LLNL address two special applications of flywheel energy storage in modular electromechanical batteries (EMBs). The first of these is the bulk storage of electrical energy for a variety of applications, including energy generated by solar and wind power systems. The second application is energy storage for vehicular uses. These two applications share the requirement that they should have minimal parasitic losses, that is, they must be capable of holding their charge for a period of days (for bulk storage systems) to weeks (for vehicular storage systems when the vehicle is not in use). In addition, the passive bearings of a vehicular system, when the vehicle is in use, must be stiff enough to be able to withstand substantial accelerations. The new concepts address both of these issues.
As is well known, any successful magnetic bearing system must be able to deal with the consequences of Earnshaw's Theorem. In essence this theorem asserts the impossibility of achieving the stable levitation of an object employing only the static attracting or repelling forces of permanent magnet elements. No matter how one arranges such magnets Earnshaw's Theorem guarantees that the will be some perturbation, involving either axial, radial, or tilting displacements, that will grow unstably. So-called “active” magnetic bearings overcome Earnshaw's Theorem by introducing sensors, feedback amplifiers, and control magnets that together act to suppress the instability.
The LLNL passive magnetic hearing systems employ dynamic effects to overcome Earnshaw's Theorem. That is, they typically consist of an array of permanent magnets to provide levitation and/or centering forces, backed up by “stabilizers” that employ repelling forces associated with currents induced in stationary windings by the time-varying magnetic fields from magnet arrays (typically Halbach arrays) that are attached to the rotating system. Since such a system is unstable when at rest, means are provided for mechanical support that then disengages when the system is rotating.
The general prescription for achieving stable levitation in such a passive bearing system is that the levitating magnet system is designed so that it is intrinsically stable (i.e., possesses positive stiffness) for two of the three perturbations: axial, radial, or tilt. The remaining unstable perturbation is then rendered stable by using a stabilizer whose positive stiffness against that perturbation is greater than the negative stiffness of the levitating magnets against that same perturbation.
It is desirable that additional constraints on the bearing system be addressed in particular applications. Examples of such constraints are applications where the positive stiffness requirements may vary during operation of the system. An example would be a flywheel system for use in a vehicle. When the vehicle is at rest, the net positive stiffness need only be sufficient to insure stability of the EMB rotor in the absence of accelerations. It is desirable to provide a stabilizer that minimizes the resistive losses in the windings when in the standby mode, thus allowing long self-discharge times. When the vehicle is in motion, however, it is desirable that the stabilizer stiffness be increased to maintain centering under the accelerations that are encountered.